In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been, and continues to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the comers and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist is then developed with a solvent called a developer which makes the exposed pattern either soluble or insoluble depending on the type of photoresist (i.e., positive or negative resist). The soluble portions of the resist are then removed, thus leaving a photoresist mask corresponding to the desired pattern on the wafer for further processing.
Projection lithography is a powerful and important tool for integrated circuit processing. As feature sizes continue to decrease, optical systems are approaching their limits caused by the wavelengths of the optical radiation being utilized. A recognized way of reducing the feature size of circuit elements is to lithographically image them with radiation of a shorter wavelength. "Soft" x-rays (a.k.a, extreme ultraviolet (EUV) radiation) having a wavelength range of about 50 to 700 Angstroms (i.e., about 5 to 70 nm) is now being investigated as an alternative photolithography methodology for next generation integrated circuits in an effort to achieve the desired smaller feature sizes.
EUV lithography may be carried out, for example, in an EUV lithography system 10 as illustrated in prior art FIG. 1. EUV radiation 11 is generated, for example, by a light source 12. A reflective condenser optical system 20 collects the EUV radiation 11 and projects the radiation 11 onto a reflective reticle 22.
The reticle 22 contains a pattern which is to be transferred to a photoresist-covered substrate 24. The reflective reticle 22 reflects a portion of the EUV radiation 11 and absorbs another portion of the EUV radiation 11 corresponding to the pattern thereon. The reflected EUV radiation 11 therefore carries the desired pattern to a reflective imaging system 28 (e.g., a series of high precision mirrors) which projects a de-magnified or reduced image of the reticle pattern onto the resist-coated substrate 24. The entire reticle pattern is generally exposed onto the substrate 24 by scanning the reticle 22 and the substrate 24 (e.g., a step and scan exposure system).
The reticle 22 of prior art FIG. 1 is an important component in the EUV lithography system 10. Unlike conventional UV lithography systems which predominately use refractive optics, many EUV lithography systems, such as the system 10 of prior art FIG. 1, utilize reflective optics. Consequently, the reticle 22 is a reflective reticle and therefore reflects the incident EUV radiation 11 to form a pattern as opposed to transmitting portions of the radiation therethrough. An exemplary reflective reticle 22 is illustrated in prior art FIG. 2. The reflective reticle 22 includes a substrate 40 such as silicon or glass having a reflective layer 42 formed thereon. The reflective layer 42 is typically a multilayer coating which is designed to reflect the EUV radiation with a high efficiency (e.g., about 65% or more). The reflective layer 42 is covered with a buffer layer 44 (e.g., silicon) which may be used to protect the reflective layer 42 and help prevent oxidation of the reflective layer 42 during subsequent processing or pattern repair. Lastly, a thin layer of EUV absorptive material (e.g., silver, tungsten, gold, tantalum, titanium, lead, polyimide, polymethyl methacrylate (PMMA), etc.) is deposited and patterned to form the desired reticle pattern 46.
The reticle blank, consisting of the substrate 40 and the reflective coating 42, is an important component in the EUV lithography system 10. The reflective coating 42 typically contains many individual reflective layers (and for this reason is often called a multilayer film or coating) which must be substantially defect free in order to provide the high reflectance needed to provide a high-throughput, cost-effective lithography system. The reflective coating 42 may include, for example, eighty (80) layers of alternating molybdenum and silicon, each having a thickness of about 3-4 nm. Because the reflectivity of the reflective coating 42 is an important performance parameter and since defects in the multilayer coating 42 negatively impact the reflectivity, the manner in which the reticle blank is fabricated is important.
In the formation of conventional reflective reticle blanks for EUV lithography systems, a performance trade-off exists. In order to provide defect-free, highly reflective coatings for the reflective layer 42, each layer within the multilayer film 42 must have a low surface roughness (e.g.,within about .+-.0.1 nm). Consequently a smooth substrate 40 such as silicon is highly desirable in order to facilitate such film thickness control during the formation of the individual reflective layers. In addition, because the reflective coating 42 only reflects about 65% of the EUV radiation, the remaining radiation is absorbed by the reticle 22 which causes the reticle 22 to heat up during processing. Consequently, in order to reduce distortion and performance degradation associated with the reticle heating (e.g., pattern overlay error), a low thermal expansion substrate such as an ultra low expansion glass is desirable.
Silicon, which facilitates the formation of high quality multilayer coatings due to its flatness, unfortunately exhibits a large coefficient of thermal expansion (e.g.,up to 2 ppm/.degree. C.) and thus exhibits undesirably large distortion due to the absorption of EUV radiation during processing. For example, across a typical 100 mm image field, a 0.5.degree. C. deviation in temperature will result in a registration error of 100 nm, in circumstances where less than 10 nm is desired. Likewise, the ultra low expansion glass, although not experiencing substantial distortion during processing, does contain an undesirably high number of surface defects (e.g., scratches) which undesirably create defects in the multilayer reflective coating 42 during the formation of the coating. Because the thickness of the various individual layers within the reflective coating 42 are preferably controlled within about .+-.0.1 nm, and the defects on the glass substrate surface may be greater than 0.1 nm, the substrate surface defects create noise in the various reflective layers, thus substantially degrading the reflectivity of the reflective coating 42. Therefore the prior art reticle blanks exhibit a performance trade-off between smooth substrates for high quality reflective layers provided by silicon substrates and low thermal expansion characteristics for minimized distortion during processing provided by glass substrates.
Therefore there is a need in the art for EUV reticle blanks which provide high quality reflectivity with minimized distortion during processing exposure.